Alzheimer's disease (AD) is a neurodegenerative disorder characterized by a progressive loss of cognitive function and is the most frequent type of dementia in the elderly, affecting almost half of all patients with dementia.
Correspondingly, advancing age is the primary risk factor for AD. Among people aged 65, 2-3% show signs of the disease, while 25-50% of people aged 85 have symptoms of AD and an even greater number have some of the pathological hallmarks of the disease without the characteristic symptoms. Every five years after the age of 65, the probability of having the disease doubles. The share of AD over the age of 85 is the fastest growing segment of the AD population in the US, although current estimates suggest the 75-84 year-old population has about the same number of patients as the over 85 population (Herbert at al., 2003). The Alzheimer's Association recently reported that there are more than 5 million people in the US living with AD (Alzheimer's Association, 2007). This number is expected to triple by the year 2050.
The current cost to government agencies of the care of patients who have AD is substantial, and it is rising rapidly: By 2010, Medicare spending on AD is expected to grow to $49.3 billion (a 54% increase over the costs in 2000), and Medicaid spending will grow to $33 billion (an 80 percent increase over costs in 2000). AD has been reported to cost American businesses $61 billion annually. Of that amount, the annual cost attributable to lost productivity and replacement costs when workers become caregivers for a relative who has AD is an estimated $36.5 billion. These costs reflect neither the direct financial costs to family caregivers (e.g., lost income) nor the costs associated with depression among family members providing end-of-life care (Prigerson, 2003). There is currently no cure for AD and available medications offer relatively small symptomatic benefit for some patients but do not slow disease progression.
AD is characterized by the deposition of cerebral amyloid-β (Aβ) protein, neuritic plaques, glial cell activation, and neurofibrillary tangles composed of hyperphosphorylated tau protein (Selkoe, 2001). Epidemiologic, pathologic, and genetic evidence demonstrates that Aβ has a pivotal role in the pathogenesis of AD (Golde, 2003). Immunization of amyloid precursor protein (APP) transgenic mice with aggregated Aβ1-42 peptide in Freund's adjuvant injected intraperitoneally resulted in the lowering of cerebral Aβ (Schenk et al., 1999). Reduced cerebral Aβ levels in PDAPP-tg mice following intranasal immunization with Aβ1-40 peptide have also been reported (Lemere et al., 2000; Weiner et al., 2000). Soon thereafter, several reports demonstrated the importance of antibody-mediated clearance of Aβ and its role in improving cognition (Bard et al., 2000; Janus et al., 2000; Morgan et al., 2000; Dodart et al., 2002). In addition, anti-Aβ antibodies have been induced following immunization with Aβ using various adjuvants (Lemere et al., 2002; Cribbs et al., 2003; Lemere et al., 2000, 2002, 2003; Spooner et al., 2002: Maier et al., 2005; Ghochikyan et al., 2006) and by DNA immunization (Ghochikyan et al., 2003, Zhang et al., 2003, Okura et al., 2006, Frazer et al, 2007). In addition to an active immunization strategy, passive immunization with antibodies against Aβ have also been shown to remove Aβ from the brain and is associated with an improvement in cognitive function in a mouse model of AD (Bard et al., 2000, DeMattos et al., 2001). Together these encouraging animal data led to a multi-center Aβ vaccine (AN1792) clinical trial (Schenk, 2002; Orgogozo et al., 2003; Gilman et al., 2005).
The AN1792 vaccine was deficient in two respects. First, AN1792 induced an effective immune response in only 59 of 300 treated patients (19.7%) and, secondly, the clinical trial had to be halted when 18 (6%) of the treated subjects experienced symptoms of meningoencephalitis (Schenk, 2002; Orgogozo et al., 2003; Gilman et al., 2005). Autopsy case reports from subjects who received AN1792 vaccination demonstrated regions with strongly reduced numbers of plaques compared to controls (Nicoll et al., 2003; Ferrer et al., 2004; Masliah et al., 2005). However, T cell infiltrates, (primarily CD4+ with fewer CD8+ cells) were present in the leptomeninges, perivascular spaces, and brain parenchyma in two cases, suggesting a T cell-mediated immune response to AN1792 vaccination. Although no neuroinflammation was observed in pre-clinical studies a recent report had shown that immunization of C57BL/6 mice with Aβ and pertussis toxin induces autoimmune meningoencephalitis with characteristics similar to those observed in humans immunized with AN1792 (Furlan et al., 2003). Follow up studies on the AN1792 trial showed that AD patients that developed antibodies that bound to Aβ plaques showed significantly slower rates of decline in cognitive function. These findings suggest that the generation of anti-Aβ antibodies by active immunization is a promising immunotherapeutic approach for AD provided that a robust immune response can be elicited in these elderly patients, and that excessive cell mediated immune reactions are minimized in order to avoid unwanted neuroinflammation. The exact mechanism by which Aβ antibodies reduce Aβ burden in the brain is not known but hypotheses include Fc-receptor mediated phagocytosis via microglia, dissolution of amyloid fibrils, or sequestration of circulating Aβ resulting in an increased net efflux of Aβ from the brain (Vasilevko and Cribbs, 2006). Clearly, whatever the mechanism, immunotherapy, either active or passive, has the potential to clear Aβ in AD and improve cognitive function.
Active immunization schedules are being developed to minimize T lymphocyte-mediated immune reactions and to maximize antibody production. The B cell epitope(s) in humans (Geylis et al., 2005), monkeys (Lemere et al., 2004) and mice (Lemere et al., 2000; McLaurin, et al., 2002; Agadjanyan et al., 2005) is located within the Aβ1-15 region, while the T cell epitope has been mapped within Aβ15-42 (Cribbs et al., 2003; Monsonego et al., 2003). Thus, Aβ fragments spanning the B cell epitope but not the T cell epitopes may be safer immunogens to potentially avoid deleterious anti-Aβ cellular immune responses. Many approaches to enhance immunogenicity of these shorter Aβ fragments have been studied such as conjugation of the B cell epitope, Aβ1-15 to the universal helper T cell epitope, PADRE (Agadjanyan et al., 2005; Ghochikyan et al., 2006), expansion with the addition of lysine residues and glutamate substitutions to reduce β-sheet content (Siguardsson et al., 2001, 2004) or presentation as multiple copies (Zhou et al., 2005). More recently an UBITh® AD immunotherapeutic vaccine has been described whereby the intrinsic self Th epitopes of Aβ1-42 are replaced with foreign UBITh® epitopes (Wang et al., 2007). Results from a repeat dose toxicity study in macaques have shown no evidence for immunotoxicity or overall toxicity following immunization with this Aβ1-14 UBITh® vaccine.
The dendrimeric Aβ1-15 (dAβ1-15), composed of 16 copies of Aβ1-15 on a branched lysine core which includes an Aβ-specific B cell epitope but lacks the T-cell epitope, is an effective immunogen. Immunization intranasally (i.n.) with dAβ1-15, using an experimental adjuvant LT (R192G), mutant E. coli heat-labile enterotoxin (Dickinson and Clements, 1995), resulted in a robust humoral immune response with a significant reduction in cerebral plaque burden in J20 APP-tg mice (Seabrook et al., 2006). When injected s.c. in mice dAβ1-15, with LT (R192G) adjuvant, induced a humoral immune response with anti-Aβ antibodies, principally of the IgG1 isotype with lower levels of IgG2a and IgG2b, which bound cerebral Aβ plaques in brain tissue from an AD patient (Seabrook et al., 2006). In another study, the i.n. immunization with a tandem repeat of two lysine-linked Aβ1-15 sequences (2× Aβ1-15) using LT (R192G) adjuvant reduced cerebral Aβ load and learning deficits in hAPP mice in the absence of an Aβ-specific cellular immune response (Maier et al., 2006).
In addition to using short Aβ derivatives that have less intrinsic neurotoxicity, adjuvants which can direct the immune response towards a Th2 phenotype may also be critical for the design of a safe, immunogenically robust, and efficacious vaccine for AD (Cribbs et al., 2003; Ghochikyan et al., 2006). QS21, an adjuvant known to induce a strong Th1 humoral response (IgG2a antibodies in mice) was used in the AN1792 trial, and may have contributed to the T cell mediated inflammation observed during its clinical evaluation. Many studies in animals to date focused on getting a good antibody titer by using strong adjuvants, such as CFA/IFA, that give a mixed Th1/Th2 immune response. It has been shown that Aβ1-15 in tandem with the universal helper T cell epitope (PADRE), PADRE Aβ1-15-MAP when formulated in alum, a Th2 biased adjuvant, generated mainly IgG1 antibody isotypes, but gave a less robust immune response than when formulated in Quil A, a Th1 biased adjuvant generating predominantly IgG2a isotypes (Ghochikyan et al., 2006). Although robust Th-2 type humoral responses to Aβ following subcutaneous or intramuscular routes have not been reported, the Th2-type humoral response in APP/TG mice following intranasal vaccination with Aβ peptide using LT (R192G) adjuvant was associated with significant decreases in the cerebral Aβ plaque burden, decreased local microglial and astrocytic activation, and reduced neuritic dystrophy (Weiner et al., 2000). Thus, in principle, an anti-Aβ vaccine that elicits a robust Th2 biased immune response in the AD population should be efficacious for treatment of AD.